The collection of samples of LNG for subsequent analysis is well known. Analysis is typically performed to determine the BTU value of the natural gas as its component compounds can vary and stratify. Historically, LNG and other forms of natural gas were sold by volume disregarding the BTU content of the natural gas. Natural gas is comprised of several component compounds and is not a “pure compound”. It may contain methane, ethane, propane, moisture and other components. Each of the components has its own BTU value, if any. As the value of hydrocarbons used to produce heat has increased, it is important to know its BTU value in order to properly price the product both from a seller perspective and a consumer perspective. LNG is transported by tanker ships, limited distances of pipeline and the like. When loading to or offloading LNG from a ship, it can be substantially continuously sampled and analyzed on site during the loading/offloading process to accommodate variation in the makeup of LNG. This process historically involves the vaporization of the LNG prior to a sample being taken or analyzed. A sample can also be taken at this point and sent away for laboratory analysis. The samples are usually taken using a sampling cylinder into which the LNG, which has been vaporized into a gas, is injected, stored and transported to an analytical facility for analysis. However, this precludes the ability to take a sample of the LNG in the liquid state, eliminating any question about what might have been altered in the vaporization process. It is important to obtain representative samples of the LNG. Because of the different components making up LNG, the obtaining of a representative sample can be difficult since the LNG can stratify if stored static, can fractionate by allowing lower boiling point fractions to evaporate or flash off from the sample. Further, if the LNG vaporizes, its volumetric increase from liquid to vapor can be on the order of a 600 fold increase in volume and result in a dramatic increase in pressure.
LNG is typically maintained at a temperature of approximately −256° F. and at a gage pressure of 40 psi to maintain it in liquid form. If this pressure is reduced or the temperature increased, vaporization of one or more components can occur. Vaporization of the various components will occur at different temperatures and pressures because they are different chemical compounds. Sampling cylinders of the piston type are well known in the art, see for example, U.S. Pat. Nos. 4,628,750 and 4,922,764, both assigned to Welker Engineering Company. The entire disclosures of these patents are incorporated herein by reference. See also U.S. Pat. No. 6,422,737 also assigned to Welker Engineering Company. This latter patent discloses the use of an integral mixing pump to maintain a more homogenous sample. The entire disclosure of this patent is also incorporated herein by reference.
While sampling cylinders are known in the art, an improvement therein is desirable to effect a more uniform and representative sample during the injection of the sample into the sampling cylinder. The present invention involves an improved sampling cylinder which allows for directly sampling LNG (liquid state) into the cylinder.